AbstractAn explicit reduced equation of state is developed by combining results of the hole'theory with a semitheoretical expression for the temperature dependence of the reduced Tait parameterB̃which is found to be in good agreement with experiment. This approach circumvents the implicit character of the theoretical equations.Two procedures are explored to derive for a given system the PVT surface and the compressional energy and entropy changes from the corresponding reduced functions. One rests on the usual superposition of experimental and analytical isotherms and isobars. The other, and more approximate method, utilizes a correlation between the scaling temperatures, pressures, and volumes obtained previously, and requires only experimental data at atmospheric pressure and a single temperature above the glass transition (Tg) or the melting temperature (Tm). The good agreement between experiment and theory noted previously is shown to extend to the semiempirical relationships. Finally, internal pressure data for oligomers and polymers at atmospheric pressure and encompasing a wide range of reduced temperatures are examined. The ensemble of data can be described by simple power relationships between the internal energy and the density. The exponents, however, depart significantly from the van der Waal's value of unity
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